Detroit Diesel Fault Code

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SPN 520371 FMI 16

MIL On / Elevated DEF Consumption

System

Emissions

Symptom

MIL On / Elevated DEF Consumption

Diagnosis & fix steps

LOGIC: SCR Closed Loop Control at Maximum Limit (Multiple Sources) / High DEF Delivery. ACTION: Test DEF concentration with refractometer (32.5% spec). Run a DEF quantity test via DiagnosticLink (spec: 108–132 mL). Perform a parked SCR efficiency test to reset tracking.

Red Seal field insights

Control Logic: The ACM continually compares NOx levels into and out of the SCR. If conversion efficiency drops, it aggressively ramps up DEF dosing. If it maxes out the dosing map volume and NOx is still high, this code sets.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

Book an uptime review

Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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